1. Field of the Invention
This invention relates to an apparatus for forming an image by means of an electrophotographic process of scanning a photosensitive member with a light spot. An image forming apparatus according to the invention can suitably be used for a laser beam printer (LBP) or a digital copying machine.
2. Related Background Art
Image forming apparatus including laser beam printers and digital copying machines are designed to use a light beam emitted from a light source such as a semiconductor laser with an intensity modulated according to the image signal applied thereto.
FIG. 1 of the accompanying drawings is a schematic cross sectional view of a principal portion of the optical scanning unit of a known image forming apparatus of the above identified type taken along the main-scanning direction. The parallel light beam emitted from a laser unit 61 containing a semiconductor laser is made to enter a cylindrical lens 62 showing a refractive power only in the sub-scanning direction. The parallel light beam entering the cylindrical lens 62 leaves it as parallel light beam in a plane intersecting the optical axis along the main-scanning direction (hereinafter referred to as xe2x80x9cmain-scanning sectionxe2x80x9d) but is converged in a plane intersecting the optical axis along the sub-scanning direction (hereinafter referred to as xe2x80x9csub-scanning sectionxe2x80x9d) to form a linear image extending in the main-scanning direction on deflection plane 63a of an optical deflector 63, which is typically a polygon mirror. The light beam reflected/deflected by the deflection plane 63a of the optical deflector 63 is then led to the surface to be scanned of a photosensitive drum 66 by way of a focussing optical system (fxcex8 lens system) 71 having an fxcex8 feature to form a light spot on the surface. Then, the surface of the photosensitive drum 66 is repeatedly scanned by the light spot. The focussing optical system 71 comprises a spherical lens 64 and a toric lens 65.
The optical scanning unit is provided with a BD (beam detector) sensor 69, which is a photodetector for regulating the timing of the image forming operation to be conducted on the surface of the photosensitive drum 66 before the light spot starts scanning the surface of the photosensitive drum 66. The BD sensor 69 receives the part of the light beam deflected by the optical deflector 63 that gets to a region outside the image forming region of the surface of the photosensitive drum 66 before scanning the image forming region. This part of the light beam is referred to as BD beam. It is reflected by a BD mirror 67 and then condensed by a BD lens (condenser lens) 68 before it enters the BD sensor 69. A BD signal (synchronizing signal) is detected from the output of the BD sensor 69 and the timing of starting the image recording operation on the photosensitive drum 66 is regulated on the basis of the BD signal.
The photosensitive drum 66 is driven to rotate at a constant rate in synchronism with the drive signal from the semiconductor laser in the laser unit 61, whereby the photosensitive surface of the photosensitive drum 66 is driven to move in the sub-scanning direction relative to the light spot scanning the surface. Then, an electrostatic latent image is formed on the surface of the photosensitive drum 66, subsequently developed by means of a known electrophotographic process and thereafter transferred onto an image receiving member such as a sheet of paper to produce a finished image thereon.
Meanwhile, the optical scanning unit is provided with an automatic power control (APC) circuit for controlling the power of the light beam emitted from the semiconductor laser, although the APC circuit is not shown in FIG. 1. The laser unit of FIG. 1 is provided with the semiconductor laser and a photodetector for detecting part of the light beam emitted from the semiconductor laser. The APC circuit samples the output of the photodetector at predetermined timing and controls the output power of the semiconductor laser on the basis of the sampled output levels.
FIGS. 2A through 2C are respective timing charts of operation of the semiconductor laser, the BD sensor and the APC circuit of a known image forming apparatus of the type under consideration for the period of a scanning cycle of the light spot that is made to scan the photosensitive drum repeatedly. FIG. 2A shows the operation of the semiconductor laser. It will be seen from FIG. 2A that the semiconductor laser is ON when the signal applied to it is at level High (H) and OFF when the signal is at level Low (L). FIG. 2B shows the operation of the BD sensor. It operates to detect the BD signal (synchronizing signal) when the signal applied to it is at level High (H). FIG. 2C shows the operation of the APC circuit. It operates to control the output power of the semiconductor laser when the signal applied to it is at level High (H).
The photosensitive drum used for the image forming apparatus has a image forming region (effective image region) and blank regions (non-image-forming regions) arranged at the opposite sides of the image forming region in the main-scanning direction. The toner image formed in the image forming region is ultimately transferred onto an image receiving member. Assuming that a light spot is made to scan the surface of the photosensitive drum in the main-scanning direction, the period during which it scans the image forming region is referred to as image forming period whereas the period during which it scans either of the blank regions is referred to as blank period. As seen from FIG. 2A, a blank period exists before and after an image forming period. Additionally, there exists a period during which the light spot scans not the surface of the photosensitive drum but outside the surface before and after each blank period. This period is referred to as off-drum scanning period.
As seen from FIG. 2A, the light beam emitted from the semiconductor laser during an image forming period is modulated by the image signal applied to it and the light spot scanning the surface of the photosensitive drum is turned on/off accordingly. The surface of the photosensitive drum is electrified in advance and, as the surface is irradiated with the light spot whose intensity is modulated, the electric potential of the surface of the photosensitive drum becomes differentiated as a function of the location on the surface to form an electrostatic latent image there. More specifically, any area on the surface of the photosensitive drum irradiated with light (exposed area) loses the electric charge whereas any area on the surface of the photosensitive drum not irradiated with light (unexposed area) retains the electric charge.
In known image forming apparatus, toner of the type showing the polarity same as the electric charge that electrifies the surface of the photosensitive drum in advance is used onto the electrostatic latent image formed thereon in a manner as described above for socalled reversal development. In other words, an exposed area becomes a printed area (area carrying the toner) because, if an unexposed area is made to become a printed area, the period for which the semiconductor laser is turned on for printing should be made relatively long.
The operation of automatic power control (APC) of controlling the output power of the semiconductor laser of the image forming apparatus is made to take place in the off-drum scanning period preceding an image forming period as shown in FIG. 2C. Then, the operation of the BD sensor for detecting a BD signal (synchronizing signal) is made to take place after the automatic power control operation also in the same off-drum scanning period that also precedes the image forming period as shown in FIG. 2B. While it is theoretically not necessary to energize the semiconductor laser in the off-drum scanning period, the semiconductor laser is in reality energized in the period during which the APC operation and the BD signal detecting operation take place as shown in FIG. 2A. The semiconductor laser is not energized in the blank period because the toner adheres to the blank region if semiconductor laser is energized in that period. While the blank region is not the area from which the toner is transferred to the image receiving member, the toner can adhere to the edges of the image receiving member when the image is ultimately transferred thereto if a slightest time lag occurs in the image transferring operation. This is the reason why the semiconductor laser is not energized conventionally in the blank period.
Referring to FIGS. 2A through 2C, the ratio of the image forming period relative to the overall scanning period of the light spot is referred to as scanning efficiency. The scanning efficiency should be high for a high definition image forming apparatus that is required to operate at high speed. However, the time allocated to the APC operation is reduced to make the operation less stable if the scanning efficiency is made to high.
Meanwhile, Japanese Patent Application Laid-Open No. 4-247477 describes an image recording apparatus in which BD signals are detected during the blank period before the image forming period and APC operation is conducted during the blank period after the image forming period. However, in the apparatus described in Japanese Patent Application Laid-Open No. 4-247477, the semiconductor laser is energized during the blank periods and accordingly, a light shielding member must be provided between the optical deflector and the photosensitive drum so that the light beam from the semiconductor laser may not get to the photosensitive drum, which will result in a complicated structure and interfere with realizing a compact device. Further, even a slight misarrangement of the light shielding member can expose the blank region partially and cause adhesion of toner on the exposed area. Then, the toner adhering to the blank region can be transferred to an edge of a sheet of paper if the positional relation between the drum and the sheet of paper is not accurate.
In view of the above circumstances, it is therefore the object of the present invention to provide an image forming apparatus that affords a stable automatic power control (APC) operation to be conducted without complicating the constitution and sacrificing the scanning efficiency.
According to the invention, the above object is achieved by providing an image forming apparatus comprising:
a light source adapted to emit a light beam modulated according to the image signal applied thereto;
an optical deflector for repeatedly deflecting the light beam emitted from the light source;
a focussing optical system for focussing the light beam deflected by the optical deflector on a surface to be scanned to form a light spot there, the surface to be scanned being scanned repeatedly in the main-scanning direction;
a photosensitive member arranged on the surface to be scanned and exposed selectively to said the spot, the photosensitive member having an image forming region and blank regions arranged at the opposite ends of the image forming region in the main-scanning direction;
a motor for driving the photosensitive member to move in the sub-scanning direction perpendicular to the main-scanning direction;
a developing unit for causing toner to adhere to the unexposed areas of the selectively exposed photosensitive member and developing an image;
a photodetector for receiving part of the light beam emitted from the light source;
an automatic power control circuit for controlling the output power of the light source on the basis of the output of the photodetector; and
a drive circuit for driving the light source, the drive circuit being adapted to drive the light source according to the image signal applied thereto during the period of scanning the image forming region of the surface of the photosensitive member by the light spot and energize the light source during the entire period of scanning the blank regions of the surface of the photosensitive member by the light spot;
the automatic power control circuit controlling the output power of the light source during the period of scanning the blank regions of the surface of the photosensitive member by the light spot.